'Grey's Anatomy': What's Owen Hiding?Kevin McKidd talks with THR about what to expect from Thursday's midseason finale.Hell hath no fury like a doctor with a bone to pick.

On ABC's Grey's Anatomy, typically calm, cool and collected doc Owen Hunt (Kevin McKidd) has become anything but now that a ghost from his past has resurfaced at Grey Sloan Memorial Hospital.

To say there's a wedge between Dr. Nathan Riggs (Martin Henderson) and Owen would be an understatement. Owen has had it out for Riggs since the latter first arrived — with a patient to visit April (Sarah Drew) — and has accused Bailey (Chandra Wilson) of putting the hospital at risk by hiring a total stranger and failing to do a thorough background check.

But the bigger question remains: Why does Owen have such a deep-rooted hatred for Riggs? His disgust with the new doc is so intense that even Meredith (Ellen Pompeo) has shunned Riggs based only on longtime friend Owen's word.

So what's behind all the animosity? Part of that mystery will be revealed during Thursday's midseason finale, when yet another person from Owen's past returns to the Seattle hospital, adding fuel to Owen's fire. (Previews have shown Owen again losing his cool and punching Riggs in the face.) THR turned to McKidd to get the scoop on what to expect from the episode.

Owen has a secret and it's weighing on him to the point where it appears his PTSD is coming back. How will he attempt to control his feelings?He's struggling to control his feelings right now. Whatever it is, it's really igniting Owen in a big way and a way we haven't seen in quite a few years. It's fun to play that again. He's been a very controlled chief for the hospital for the past four or five seasons so it's nice to see that fiery side of Owen come back. He doesn't cope particularly well. One of the things I love about Owen is that he makes messy choices and doesn't deal with big emotions particularly well. He's very male in that sense. And it climbs up to the point where he acts out. It's interesting to see how badly he is doing right now. (Laughs.) I feel for him! There's obviously something really deep at the core that happened between Owen and Riggs.

There's a history between Owen and Nathan. How surprising will what caused their rift be? It will be surprising. I was surprised when I found out what it is. Some of the story points are still being hammered out. It illuminates more of Owen's backstory and where he's from and what his psychological make-up is that helped him become the person he is today. It's cool to play some conflict between male characters, and we haven't had that for a while. It's been very female-centric for a long time — as it should be — but it's nice to see two men really clash in a bit of life-and-death heated kind of way.

Someone from Owen's past is returning in this episode. What can you say about who that is? How will their arrival help flesh out what's going on between Owen and Nathan? It's someone Owen loves deeply. There are a few more pieces of the jigsaw puzzle that get put in place when that person shows up. You start to get a sense of what might have happened, but the midseason finale definitely poses more questions than answers. You get a few tidbits but it leads to more questions and you'll have to wait until next year to find out.

Owen won't even tell Amelia what's going on with him. Why is he keeping her in the dark? They're very connected and he has shared a lot with her — and she has shared a lot with him — but this thing is so unusual and so dark for Owen that he can't tell anybody about right now. He doesn't know which way is up and he's not doing very well in that department. He's not giving Amelia the trust that she deserves. She wants him to lean on her but he doesn't want to do it. Owen isn't acting in his right mind and isn't being true to the relationship. But hopefully they'll get through it. It's definitely a roadblock for them and will cause problems for them. But at the end of the day, Owen will eventually look up to her.

Riggs is attempting to buddy up to both Meredith and Maggie (Kelly McCreary). Is he targeting them for a reason? He's looking for allies at this point — as anybody would in his position. He made his bed and he wants to be in this place and got this job and everybody in the hospital is family at this point. So for Owen to say, "I hate this guy; nobody trust this guy," that's tough for Nathan to come back from. He's looking for any support he can, which is a very human instinct in a situation like this. I don't blame him but I wish him luck! Martin is doing a fantastic job. It's tricky for both of us because we have to play this tension and conflict before they told us what the reasons for it was. We had to just have faith in Shonda Rhimes and the Grey's writers. It's pretty cool for an actor to have to play conflict from a past issue that you don't know the the nature of yet. It's like a trust exercise.

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